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Sodium tungstate-catalyzed epoxidation of α,β-unsaturated acids using H2O2 in water was first demonstrated by Payne in 1959 and was followed up by Sharpless in 1985 (G. B. Payne, et al., J. Org. Chem. 1959, 24, 54; K. S. Kirshenbaum, et al., J. Org. Chem. 1985, 50, 1979). Tungstates are known to be polymeric in aqueous solutions and the distribution of the polyoxotungstate species is dependent on pH and concentration. Peroxotungstate complexes are known to be the catalytic species in these reactions (K. A. Jørgensen, Chem. Rev. 1989, 89, 431; M. H. Dickman, et al., Chem. Rev. 1994, 94, 569; N. Mizuno, et al., Coordin. Chem. Rev. 2005, 249, 1944). Investigation by Venturello into the role of phosphate in phase transfer tungstate oxidation reactions, resulted in the isolation and identification of the heteropolyperoxotungstate, [PO4{WO(O2)2}4]3− (C. Venturello, et al., J. Org. Chem. 1983, 48, 3831; C. Venturello, et al., J. Mol. Catal. 1985, 32, 107). This peroxotungstate species was also postulated to be the catalytically active species for the H3PW12O40/H2O2(Keggin's reagent) oxidation system developed by Ishii (Y. Ishii, et al., J. Org. Chem. 1988, 53, 3587; A. J. Bailey, et al., J. Chem. Soc., Dalton. Trans. 1995, 1833; D. C. Duncan, et al., J. Am. Chem. Soc. 1995, 117, 681) Subsequently, Noyori developed an efficient catalyst suitable on a practical scale with high turnover number. It was found that (aminomethyl)phosphonic acid or phenylphosphonic acid was effective in accelerating the reaction. It was proposed that a 1:1 complex between phosphonic acid and monoperoxotungstate is the active catalyst (R. Noyori, et al., Chem. Commun. 2003, 1977). Using this methodology, they furnished olefin epoxidation (K. Sato, et al., J. Org. Chem. 1996, 61, 8310) and sulfoxidation (K. Sato, et al., Tetrahedron 2001, 57, 2469) in high chemoselectivities.
Molybdenum-based systems have been extensively applied in the field of inorganic, organic and biological chemistry (J. Burke & E. P. Carreiro. in Comprehensive Inorganic Chemistry II (Second Edition), 309-382 (Elsevier, 2013)). Molybdenum metalloenzymes play an important role in the metabolism of nitrogen (Yoshiaki Nishibayashi, Inorg. Chem. 54, 9234-9247 (2015)), sulfur, and carbon compounds (R. Hille, et al., Chem. Rev. 114, 3963-4038 (2014); Barbara K. Burgess & David J. Lowe, Chem. Rev. 96, 2983-3012 (1996); Günter Schwarz, et al., Nature 460, 839-847 (2009)). Over recent years, various molybdenum compounds have been developed and successfully applied in a number of organic transformations. In particular, the properties of the oxomolybdenum (VI) anionic species have been comprehensively investigated and described. It is worthy of note that the reactions with organic ligands, strong acids and oxidants allow the formation of numerous ionic complexes of molybdenum. A few heteropolymolybdate complexes (Alan J. Bailey, et al., J. Chem. Soc., Dalton Trans., 1833-1837 (1995); N. Melanie Gresley, et al, J. Mol. Catal. A: Chem. 117, 185-198 (1997); Karl-Heinz Tytko & Dieter Gras, (Springer Berlin Heidelberg, 1988) involving nitrate, fluoride, chloride and phosphate groups (Li Mingqiang & Jian Xigao, Bull. Chem. Soc. Jpn. 78, 1575-1579 (2005)) have been investigated to afford more structural and catalytic diversity.
Molybdate ions can act as catalysts for the activation of H2O2 by forming monomeric or polymeric peroxomolybdates, which are highly dependent on pH value of the solution and the quantity of H2O2(Valeria Conte & Barbara Floris, Dalton Trans. 40, 1419-1436 (2011); Michael H. Dickman & Michael T. Pope, Chem. Rev. 94, 569-584 (1994)).
The coordination pattern and fine structure of peroxomolybdate anions and corresponding counter cations can have a significant impact on their performance as oxidizing reagents (Xianying Shi & Junfa Wei, Appl. Organomet. Chem. 21, 172-176 (2007)). Recently, the coordination chemistry of anionic peroxomolybdate species with different organic ligands such as citric and malic acids (Zhao-Hui Zhou, et al., Dalton Trans., 1393-1399 (2004)), amino acids (Katarzyna Serdiuk, et al., Transition Met. Chem. (London) 26, 538-543) and oxalic acid (Andrew C. Dengel, et al., J. Chem. Soc., Dalton Trans., 991-995 (1987); Rajan Deepan Chakravarthy, et al., Green Chem. 16, 2190-2196 (2014)) have been systemically investigated. However, the protocol for preparation of peroxomolybdenum complex with a sulfate ligand is limited (Chang G. Kim, et al., Inorg. Chem. 32, 2232-2233 (1993); Masato Hashimoto, et al., J. Coord. Chem. 37, 349-359 (1996); Fabian Taube, et al., J. Chem. Soc., Dalton Trans., 1002-1008 (2002); Dao-Li Deng, et al., WO2006094577A1 (2006)) only one example using such a system for the catalysis of olefin epoxidation reaction has been reported so far (Laurent Salles, et al., Bull. Soc. Chim. Fr. 133, 319-328 (1996)).
The preparation of enantiopure chiral sulfoxides is an important field because new and better methods will enable more convenient access to potential drug molecules (for selected reviews, see: a) I. Fernández, N. Khiar, Chem. Rev. 2003, 103, 3651; b) H. B. Kagan, T. O. Luukas, Transition Metals for Organic Synthesis: Building Blocks and Fine Chemicals, Second Revised and Enlarged Edition. 2004: 479; c) H. B. Kagan, Wiley-VCH: Weinheim, Germany, 2008; d) G. E. O'Mahony, A. Ford, A. R. Maguire, J. Sulfur Chem. 2013, 34, 301).
Currently, the Kagan oxidation is widely used for asymmetric sulfoxidation (H. B. Kagan, F. Rebiere, Synlett 1990, 11, 643) but there are emerging methods (F. A. Davis, R. T. Reddy, W. Han, P. J. Carroll, J. Am. Chem. Soc. 1992, 114, 1428; J. Legros, C. Bolm, Angew. Chem. Int. Ed. 2004, 43, 4225; Angew. Chem. 2004, 116, 4321; C. Drago, L. Caggiano, R. F. W. Jackson, Angew. Chem. Int. Ed. 2005, 44, 7221; Angew. Chem. 2005, 117, 7387; J. Fujisaki, K. Matsumoto, K. Matsumoto, T. Katsuki, J. Am. Chem. Soc. 2011, 133, 56) to prepare chiral sulfoxides including some recent breakthroughs utilizing imidodiphosphoric acid (S. Liao, I. Čorić, Q. Wang, B. List, J. Am. Chem. Soc. 2012, 134, 10765), binuclear titanium chiral complex (S. Bhadra, M. Akakura, H. Yamamoto, J. Am. Chem. Soc. 2015, 137, 15612), or pentanidium (L. Zong, X. Ban, C. W. Kee, C.-H. Tan, Angew. Chem. Int. Ed. 2014, 53, 11849; Angew. Chem. 2014, 126, 12043).
There remains a need for new methods of accessing chiral sulfoxides in high enantiopurities and for catalysts/catalyst systems that can be used to accomplish these as current methods may not be able to work with particular substrate materials of significant interest in the field of pharmaceuticals and the like.